Can you export a mp4 as .webm with the new Blender 2.8 update?
Update 2

Lukasz-40sth is right Blender 2.81 has a WEBM container which eliminates all the previous issues. It also supports the Opus audio codec. These features were added by Dr. Sybren Stüvel based on FFmpeg which implements the container and codecs.





Update

While the output described in the configuration below does output a video that is playable by all major browsers, it doesn't follow the guidelines for WEBM. For instance the WEBM Container Guidlines specify that:


  DocType element SHOULD be "webm"


However Blender's output has DocType set to matroska when using the Matroska container. I assume that SHOULD in this context refers to the meaning specified in RFC 2119, it's recommended but not a must. The file should therefore still be a valid WEBM.



Disclaimer: I'm not very familiar with video container formats, the following information may be inaccurate.

Blender 2.80 doesn't have a dedicated WEBM container, however WEBM is a subset of the Matroska container format that only allows a specific set of audio and video codecs. Therefore you should be able to produce a valid WEBM with the following settings:


Container   : Matroska
Video Codec : WEBM / VP9
Audio Codec : Vorbis


The output file will have the .mkv extension which you can rename to .webm.





When you have created your video in the VSE and then use Render Animation to export the video, it will be stored in the directory that is specified in the Output panel (for example in the screenshot it's /temp). Make sure that you're checking in the correct directory.